The goal of the proposed research is to increase our understanding of the function of the vestibular otoliths in the vestibulo-ocular reflex (VOR). Semicircular canal and otolith signals are processed by vestibulo-ocular neurons and routed to the extraocular muscles to rotate the eyes in the direction opposite to head rotation. The vestibulo-ocular reflex thus maintains a constant direction of eyesight during head movement, as required for acute vision. Neural signals from the otoliths are especially important for gaze stability during vertical head movements. The proposed research will characterize vision-stabilizing eye movements produced by otoliths and explore underlying neuronal mechanisms in four series of experiments in squirrel monkeys, cats, and rats. (l) Vestibulo-Ocular Reflex during slow rotations: Eye movements and single vestibular neuron activity will be recorded during low frequency and constant velocity pitch vertical rotations in upright, on-side, and inverted head posture, to isolate vestibulo-ocular reflex signals related to otoliths. Neuronal and reflex eye movement signals will be classified as due to tilt stimulation or central nervous system estimation of head velocity. Neurons identified as projecting to extraocular muscle motor nuclei will be tested to determine whether. they carry selective components of the vestibulo-ocular reflex signal or overall motor commands. (2) Selective roles of anterior canal and otolith signals: Properties of identified brainstem vestibular nucleus neurons receiving primary input from anterior semicircular canals will be compared to those with posterior canal input, to determine if otolith-based velocity estimation signals are selectively present on anterior canal neurons and otolith-based tilt signals selectively present on posterior canal neurons. (3) Signal gating in vestibulo-ocular reflex neurons: Control of otolith signals participating in vestibular reflexes will be tested by recording vestibulo-ocular reflexes in responses to fore-aft linear motion, in monkeys gazing in different directions. Identified vestibular nucleus neurons will be classified as vestibular stimulus- linked or ocular reflex response-linked, depending on whether their responses are inverted when the ocular reflex direction is reversed with reversal of gaze direction. (4) Modeling tilt, estimator, and canal signals for vestibulo-ocular reflex: Results obtained under aims 1-3 will be incorporated in models of vestibular reflexes in an attempt to develop comprehensive models that accurately mimic vertical VOR responses. The results will add to our knowledge of gaze stability and may give insight into general brain mechanisms of sensory- motor systems.